1. Field of the Invention
The present invention relates to loudspeakers, and particularly relates to compound loudspeakers, that is, loudspeakers comprising at least two acoustically radiating diaphragms.
2. Description of Related Art
Compound loudspeakers have been known for many years. For example, U.S. Pat. No. 5,548,657 (KEF Audio (UK) Limited) discloses a compound loudspeaker comprising an acoustically radiating dome-shaped high frequency diaphragm and an acoustically radiating low frequency conical diaphragm. The compound loudspeaker illustrated in U.S. Pat. No. 5,548,657 is shown in FIG. 1 of the present accompanying drawings. The two diaphragms of the loudspeaker 1 are substantially coaxial and the low frequency conical diaphragm 3 is situated radially outwards of the dome-shaped high frequency diaphragm 5. A narrow annular air gap 7 is present between the neck 9 of the conical diaphragm 3 and the external diameter of an annular baffle 11 surrounding the dome-shaped diaphragm. This gap provides a passage for air between the inside and the outside of the loudspeaker cabinet (the cabinet is not illustrated, but in practice encloses the periphery and rear of the compound loudspeaker). The gap needs to be narrow to ensure that the high frequency response of the dome-shaped diaphragm is unaffected by diffraction from the gap (the gap being a discontinuity). However, in some circumstances, for example if the cabinet of the compound loudspeaker is small, and the loudspeaker is operated at low frequencies, the difference in air pressures between the interior and the exterior of the cabinet can be great. When the low frequency diaphragm is operated at large excursions (i.e. large forward and back sound-generating motions), the air pressure differential can be sufficient to force air to flow through the gap, causing audible turbulent airflow, which clearly is undesirable.